There exists a fundamental gap in the knowledge base regarding molybdenum cofactor sulfurase C-terminal (MOSC) domain protein structure and function, as well as the role of the complex pyranopterin dithiolene ligand in the catalytic cycles of all pyranopterin molybdenum enzymes. Our long-term goal is to develop a molecular electronic structure understanding of pyranopterin molybdenum enzyme mechanism and function in order to have a positive impact on the quality of human health. Our primary objectives are to determine 1) electronic structure contributions to pyranopterin dithiolene function, 2) geometric and electronic structure contributions to the reactivity of MOSC domain proteins, and 3) why dimethyl sulfoxide reductase family enzymes require two pyranopterin dithiolenes in order to function. We will accomplish these objectives by using a combined spectroscopic approach (electronic absorption, MCD, Raman, XAS, EPR, etc.) augmented by vibrational, spectroscopic, bonding, and reaction coordinate computations to probe the pyranopterin and catalytic mechanisms. Parallel studies on judiciously chosen small molecule analogs will complement this approach. The central hypothesis is that Mo site geometric and electronic structure, coupled with the nature of the pyranopterin dithiolene, contributes to the unique function of these enzymes. The rationale for this research is that a comprehensive understanding of MOSC family proteins and the complex interplay between the Mo ion and the pyranopterin dithiolene will provide new insights into disease states and have a positive impact on human health. We will test our central hypothesis in order to accomplish the stated objective of this proposal through the successful pursuit of three Specific Aims 1) Determine how the pyranopterin contributes to electron transfer and redox processes in xanthine oxidase and sulfite oxidase family enzymes, 2) Understand structural and mechanistic relationships between MOSC domain proteins that define their function, and 3) Develop an understanding of how coordination number and the cooperative relationship between two different pyranopterins contribute to catalysis in dimethyl sulfoxide reductase family enzymes. Our contribution is expected to provide a detailed understanding of MOSC mediated catalysis at the molecular level, the link between pyranopterin dithiolene oxidation state and the role of the pyranopterin dithiolene in catalysis, and the relationship between coordination environment and the need for two pyranopterin dithiolenes in dimethyl sulfoxide reductase family enzymes. The proposed research is significant since it will advance our understanding of post-translational sulfuration processes and molybdenum cofactor trafficking, mechanisms of prodrug activation and xenobiotic detoxification, and the electronic flexibility of the pyranopterin dithiolene in molybdoenzyme catalysis.